"All of this leads to scheduled ops being cancelled and waiting times rocketing."

Welsh Liberal Democrat leader Kirsty Williams said: "The [health] minister needs to be looking at why, after one month of achievement in October, response times are once again on a downward spiral.

"It's a sad fact that the Welsh Labour government is failing patients across Wales.

"Whether it is inadequate cancer care, poor ambulance response times or dire A&E waiting times, the Welsh Labour government is not offering a health service that patients in Wales deserve."

Elin Jones, who speaks for Plaid Cymru on health, said: "The government had claimed that the service was 'on a solid basis' for winter back in October when the service hit its target for the first time in over 18 months.

"Since then, the deterioration is worrying and suggests that improvements have still yet to happen despite assurances that they would.

'Spikey' demand

This is a reflection of pressure across the whole unscheduled care systemStatement, Welsh government

"It's also very concerning that over 340 ambulances took longer than 30 minutes to reach the scene," she added.

In a statement, the Welsh government said: "We are encouraged to note ambulance performance in December 2013 was higher than the same period in 2012.

"The ambulance trust experienced increased pressure in December with unpredictable and 'spikey' demand causing random peaks in the number of acutely ill patients requiring an emergency ambulance response.

"This is a reflection of pressure across the whole unscheduled care system.

"The figures show that 67.9% of cat A calls received response within 10 minutes, 83.6% within 15 minutes and 91.9% within 20 minutes.

"This is testament to the hard work of paramedics and ambulance trust staff who are often criticised for failing to achieve a target that is only very weakly backed by clinical evidence as a measure of best outcomes for patients," the statement added.